The present invention relates generally to methods and apparatus adapted to disable the actuation assembly for a perforating gun or another detonation device used in subterranean wells; and, more specifically, relates to methods and apparatus for performing such disabling through use of a material which changes state under generally predetermined or known conditions.
As is well known in the art, a perforating gun is utilized to perforate well casing, or other oil field tubular members, and the surrounding environment, to facilitate the flow of fluids from external to the casing to the interior of the casing. The environment surrounding the casing will typically include concrete sheeting as well as the earth formation itself. In present times, the perforating is typically performed through detonation of explosive shaped charges.
Because of the forces generated during detonation of a perforating gun, a major concern in the industry has always been the avoidance of any accidental or untimely detonation of the perforating gun. For example, a detonation of a perforating gun at the surface of the earth is likely to cause significant damage to property in the vicinity of the perforating gun, and serious injury, if not death, to persons in the vicinity.
Downhole explosive devices, such as a perforating gun, are typically actuated through firing heads which are responsive to either mechanical forces or fluid pressure. So-called mechanically actuated firing heads are typically responsive to an impact such as may be provided by the dropping of a detonating bar through the tubing to impact an actuation piston in the firing head. So-called "hydraulically-actuated" firing heads are responsive to a source of fluid pressure, such as in either the well tubing or the well annulus, which will move an actuation piston in the firing head to initiate detonation of the perforating gun. Additionally, some hybrid systems exist, wherein a mechanical impact will be used to release the firing head, while an actuation piston will actually be moved by fluid pressure. An example of this type system is disclosed in U.S. Pat. No. 4,911,251, issued Mar. 27, 1990, to Flint George et al., and assigned to the assignee of the present invention. Such firing heads, where the piston is moved in response to hydraulic pressure, are believed to enhance the safety of the detonating system in that they are unlikely to detonate without a specific source of substantial fluid pressure. Such a source of fluid pressure would be expected to be found only within the wellbore.
In one attempt to provide a safety mechanism for a mechanically-actuated firing head, one company has proposed the use of an eutectic alloy placed beneath the head of the impact piston and the body of the firing head. Upon melting, the alloy will flow from beneath the piston in the firing head. The expectation is that the alloy, which forms a restraining block, will prevent substantial movement of the impact piston when the alloy is in a solid state, but will allow movement of the firing pin when the alloy is in a liquid state. The alloy is selected to change state from solid to liquid at a temperature which is less than the temperatures to which the perforating assembly will be exposed within the wellbore. Accordingly, upon temperatures exceeding the threshold temperature, or "melting temperature," at which the change of state occurs, the firing pin would be moveable in response to a mechanical impact. A paper describing the system is that identified as "SPE #22556 Three New Systems which Prevent Firing of Perforating Guns and String Shots On or Near the Surface", presented for SPE publication July 1991, by J. V. Carisella, Sc.D. and R. B. Cook, High Pressure Integrity, Inc., and J. E. Beardmore, Jr., Marathon Oil Company.
A problem with such system, however, is that design compromises must be evaluated relative to providing a large enough block to prevent a movement of the impact piston which would be sufficient to detonate the ignition charge, but which is not so large as to provide either an unrealistic barrier to movement of the firing pin even when in the liquid state or which would take an unreasonably large amount of time to change state to a degree sufficient to allow movement of the firing pin.
In addition, when the conventional system is inserted in the wellbore and is later withdrawn before the ignition charge has been detonated, as is not uncommon, the effectiveness of the conventional safety mechanism is greatly diminished. This is particularly true when the conventional system is not substantially vertically oriented when it is down the wellbore: i.e., when the conventional system is inserted into the string of tools in an upside down configuration (as is often done to provide a secondary means of detonating the perforating gun should the primary means fail) or when the conventional system is inserted in a deviated wellbore.
Accordingly, the present invention provides new methods and apparatus whereby detonation is interrupted whenever the firing head assembly or other detonating assembly is not in the wellbore. However, detonation is uninterrupted whenever the assembly is in the wellbore at a sufficient depth. Thus, the problems associated with the conventional safety mechanism are avoided.